Antimicrophonic filament



1942- H. K. ISHLER I ,277

ANTIMIGROPHONIC FILAMENT Filed June 15, 1941 INVENTOR Patented Nov. 24, 1942 ANTIMICROPHONIC FILAMENT Harry Kenneth Ishler, Emporium, Pa., assignor to Sylvania Electric Products Inc., a corporation of Massachusetts Application June 13, 1941, Serial No. 397,856

6 Claims. (Cl. 25027.5)

This invention refers to electron discharge devices and in particular to thermionic amplifier tubes with heatable filaments.

In electron tubes employing filamentary cathodes, in order to keep the filamentary cathodes from buckling when heated, they are kept under tension. Because of this tension the filaments behave like elastic strings, and have natural vibration periods at which they are especially susceptible to any shock to which the tube may be exposed. The filament, considered as a stretched string, will be very little damped, because it vibrates in vacuum. Any oscillation of such a filament, once set up, will therefore tend to be sustained for a long period of time.

As is well-known in the art, the various characteristics in thermionic amplifier tubes depend to a large extent on the average distance between filament and grid. Any oscillation of the filament corresponds therefore to an oscillation of tube characteristics, which will in turn result in an oscillating output of the tube even for fixed voltages applied to grid and plate. When the output of the tube is connected to a loud speaker, a vibration of the filament produces a sound with a frequency either equal to or double that of the fundamental mechanical frequency of the filament. Under certain conditions, other harmonics of the fundamental mechanical filament frequency may occur in the tube output. The phenomenon is known as microphonism and tubes that produce it are called microphonic.

Asthe described connection between micro-.

phonism and sustained mechanical vibration of a filamentary cathode was recognized rather early in the'radio art, the obvious methodof preventing microphonism at its source, viz., the application of mechanical damping means to the filament, was soon carried out in addition to shock absorbers provided toradio sets and tube sockets, preventing any shocks from reaching the filament structure of the tube.

One of the conventional methods of damping the filament of a radio tube consists in mounting a, small piece of mica at a neighboring electrode in such a position that it touches thefilament at a, point about half way between its two-fixed suspensions. This method is not very convenient for mounting, and has therefore generally been replaced by the application of one or several small metal wire hooks, coated with insulator material at the hook itself. Each hook grips the filament and is welded at its other endto the grid rod for keeping the hooks in position.

the mounting of mica vanes, has a number of disadvantages, in particular when applied to modern tube designs with the extremely small distances between electrodes, and the closely wound thin grid wires. Welding of additional wires on grid supports between the grid wires distorts the grid structure, and the insertion of filaments into the hooks is a delicate operation during which the grid may be further distorted with a corresponding deviation of tube characteristics from the established standards. Another difficulty results from the fact that, in general, insulator coatings do not adhere very firmly to the concave parts of bent wires, in particular when the radius of curvature is small, as it must be for a hook finding space for gripping the filament within the region limited by the grid of a vacuum tube. This dificulty may lead to filament grid shorts, and thus to the loss of many tubes. Finally, the mounting operation must be carried out with great care, and takes an undesirably long period of time.

Allthese difficulties are avoided according to the invention, by providing a spring clip, which can be removably clamped to the grip supports between consecutive grid wire turns. The elasticity of the clip holds it in position and in addition, the clip is prevented from sliding by being lodged between adjacent grid turns. The two end parts of the clip are so shaped as to surround a large part of the circumference of the cross section of each grid support. The middle part of the clip consists of a doubly bent section as shown in Fig. 2. This clip may be made of a thread drawn from a piece of silica rod or tubing, or of" a glass of very high silica content, in order to eliminate all danger of filament grid shorts. For practical purposes, tungsten wire is preferable, and in this case the straight middle section of the clip is coated with insulating material. Whatever material may be preferred, the method eliminates welding of the clip to the nearest grid. or any other adjacent electrode.

One end of the clip is equipped with an antenna-like wire continuation, which facilitates mounting, and may be taken off after the mounting operation.

It istherefore a mainobject of this invention to provide a filament damper for thermionic tube amplifiers, which consists of a spring clip which isheld to the grid supports of the tube by its own elasticity.

Another objectoi the invention relates to the shape ofthe filament damper, which is charac- This hook method though more practical than ter-izedby two straight pieces of insulator fibre or wire bent back against each other at the location where the damper contacts the filament.

A further object of the invention is to provide a metallic insulator coated filament damper, the design of which protects intrinsically against filament-grid shorts.

An object of the invention refers to a filament damper, which eliminates the welding operation usually necessary in mounting the conventional metallic filament dampers to neighboring electrodes.

A feature of the invention refers to a nonmetallic damping member, the presence of which has negligible influence on the tube characteristics.

Another feature relates to a filament damper which reduces to a minimum the heat transfer away from the filament at its contact point with the damper.

Another feature refers to the method of mounting of the filament in a tube in which the new damper is incorporated.

Another feature relates to the automatic locating of the displacing member during mounting, which insures the correct position necessary for performing its function without the application of jigs or gauges and without requiring specialized operator skill.

A further feature relates to an improved nonmicrophonic thermionic amplifier tube, with a filamentary cathode, which is free from undesirable variations of characteristics from tube to tube of the same type.

A still further feature relates to the simplified mounting of a non-microphonic tube with filamentary cathode which allows an increase of mounting speed of such tubes, and eliminates the danger of distorting the grid structure usually occurring during that operation.

Fig. 1 is a view of a completed tube embodying the invention, but with part of the tube broken away for clarity of illustration.

taken along the line 22 thereof.

Fig. 3 is an enlarged plan view of the filament damper according to the invention.

Fig. 4 is an enlarged perspective view of the filament damper showing its relation with the filament and grid.

Fig. 5 is a modification of Fig. 4.

Fig. 6 is a partial top-plan view of the electrode mount to show the wedge-shaped openings in the mica spacers through which the filament ends pass.

Referring now to Figs. 1 to 4, a piece of tungsten wire is bent in a plane into a shape similar to the letter W to form a flexible clip. Numerals I and 2 represent the two curved ends of the clip, which can be clamped over the grid support wires. Between these end pieces are the bridge portions 3 and 4 and a middle section 5, bent back upon itself in the plane of the paper at right angles to the line formed by bridge portions 3 and 4. Numeral 6 is a handle, which serves for holding the filament damper during the operation of mounting it on the grid supports. The whole clip is formed of a single piece of wire, the part 5 being coated with any desirable refractory insulator material.

In mounting the filament damper on the side rods 8 and 9 (Fig. 4) of grid I0, whose distance 1 measured from outside to outside isslightly larger than the distance d between the end parts of pieces I and 2 on their concave sides when the the handle 6 between two fingers of one hand, inserts end piece I around grid support 8 between two wire turns, say II and I2, of the grid and stretches the clip slightly so end 2 can enter around grid support 9, as shown in Fig. 4.

The spring clip is now firmly attached to the grid by its own elasticity, forming as it were an intermediate half turn with a finger pointing toward the other side of the grid, which finger is formed by the middle section 5 of the clip. The handle 6 is now out off near the grid support.

In the preferred method of mounting, the described operation is carried out before assembling the grid on the mount, and, in the case of a single filament strand I3, the filament is threaded through the mount as the last operation. The filament I3 is passed for this purpose either to the left or to the right of the two branches of section 5 of the clips as shown in Figs. 2 and 4. The filament is then welded at its ends to its lead wires and to a spring I4 which insures the proper tension. After this operation is carried out, the filament will be slightly displaced toward one of the side rods of the grid by section 50f the clip, over which it passes with a slight kink as shown in Fig. 1.

The filament tension is then preferably so adjusted that the filament has not too much friction at the section 5, so it still can slide comparatively easily in a longitudinal direction at right angles to this section when the filament expands or contracts, without rubbing the insulating material ofi section 5 of the filament clip. The preferred method of mounting, of course, may be replaced by a different sequence of steps, and in the case of a multi-leg filament such as a V-shaped or multiple filament, the legs thereof may be deflected by several clips with appropriately located deflector sections similar to those illustrated.

' More than one reverselybent deflector section, similar to 5, may be bent into the part of the clip between the ends I and 2. In certain tube types, the filament may be mounted before mounting the grid, depending on the construction, and the deflecting clip may be attached after mounting filament and grid on the stem.

The adjustment of the filament tension, though critical, can easily be handled by a skilled operator. In any event, this adjustment is much simpler with the new filament displacer according to the invention than with the conventional wire hooks which are'welded to a grid side rod or another neighboring electrode.

In general, the filament tension should be kept as low as possible for best results. With too high a filament tension the desired damping action is partly lost, as the damper will then rather act as an additional rigid supporting member near the middle of the filament, thus creating two undamped filament pieces of shorter length than the original undamped full length. This transition from a damping action into that of a rigid support added at the contact point, which crewithout damping depends of course, not only on the filament tension, but very considerably on the position of the displacer with respect to the rigid end supports of the filament. The filament displacer must be so formed that its active part displaces the filament from the straight line connecting its end points.

The advantage offered in-this respect by the spring clip displacer according to the invention becomes particularly evident for those tube types with a single strand filament, in which the location of the filament ends is fixed at a corner of each of the wedge-shaped holes I 5 in the top and bottom mount spacers l6 and IT. The same mount spacers also locate the ends of the grid supports, and it is obvious that the correct position of the active part of the filament damper according to the invention can be easily made self-adjusting for large production volumes once the desired location has been determined for any specific type of this class of tubes. The location of the active part of the filament displacer with respect to the filament is thus completely determined, because the filament displacer has a fixed location with respect to the grid supports, and the latter and the filament ends are uniquely positioned with respect to the mount spacers. Instead of individually insulating the spaced legs of section 5, this section can have a unitary insulation coating as shown in Fig. 5.

What I claim is:

1. A filament damper comprising a flexible wire-like member having end portions for re- :siliently gripping a pairof uprights, said member having an insulating'portion intermediate its ends for extending between the uprights to prevent short-circuiting thereof and to engage the filament frictionally.

2. A filament damper comprising a self-clamping flexible wire-like'member having end portions for resiliently gripping a pair of uprights between which the filament extends, said member having an offset insulating portion intermediate its ends for engaging the filament frictionally between its ends while preventing short-circuiting of said uprights.

3. A filament damper according to claim 1 in which each of said end portions is in the form of a hook with the distance between the hooks slightly less than the overall distance of said uprights whereby said damper is retained on said uprights by its own spring action.

4. An electron tube mount including a heatable filament, a grid electrode surrounding said electrode and having a pair of side rods, and a flexible member removably clamped to said side rods by its own resiliency and having an extension intermediate its ends engaging said filament frictionally.

5. A mount according to claim 4 in which said grid has spaced turns and said member has its ends located between adjacent grid turns.

6. An electron tube mount comprisinga pair of uprights, upper and lower insulator spacer members anchored to said uprights, each of said spacer members having a wedge-shaped opening, a heatable filament extending between said uprights and through said openings engaging an edge of each opening, means maintaining the filament under spring tension, and a flexible HARRY KENNETH 

